Nonlinear optical materials which produce nonlinearity between polarization and electric field upon application of a strong optoelectric field as by laser light are drawing the attention of researchers. Details of such nonlinear optical materials are given in many references including: "Nonlinear Optical Properties of Organic and Polymeric Materials", ACS SYMPOSIUM SERIES 233, ed. by David J. Williams, American Chemical Society, 1983; "Yuki Hisenkei Kogaku Zairyo (Organic Nonlinear Optical Materials)", ed. by M. Kato and H. Nakanishi, CMC, 1985; and "Nonlinear Optical Properties of Organic Molecules and Crystals", Vols. 1 and 2, ed. by D. S. Chemla and J. Zyss, Academic Press, 1987.
One application of nonlinear optical materials is in wavelength converting devices that use not only the second harmonic generation (SHG) based on quadratic (second-order) nonlinear effects but also sum and difference frequencies. The only commercially used nonlinear optical materials have been inorganic perovskites typified by lithium niobate. However, it has recently become clear that .pi.-electron conjugated organic compounds having both an electron donating group and an electron withdrawing group have much better performance as nonlinear optical materials than the aforementioned inorganic materials.
In order to form nonlinear optical materials having better performance, compounds having high nonlinear susceptibilities in a molecular state must be arranged in such a way that inverse symmetry will not occur. For the development of high nonlinear susceptibility, compounds having long .pi.-electron conjugate chains are known to be useful and described in the references mentioned above. However, the absorption maxima of those compounds are shifted to the longer wavelength and the transmittance of certain light, say, blue light will decrease to impede the generation of blue light as a second harmonic wave. This problem also occurs in p-nitroaniline derivatives. The significant effect of light transmittance on the efficiency of second harmonic generation is obvious from FIG. 4 on page 186 of Alain Azema, Proceedings of SPIE, Vol. 400, New Optical Materials, 1983.
It is therefore desired to develop a nonlinear optical material having high transparency to blue light. Attempts have heretofore been made to replace carbon atoms on the benzene nucleus of nitroaniline by other atoms such as nitrogen atoms but no completely satisfactory results have been attained.
More effective methods have been proposed in JP-A-62-210430 and JP-A-62-210432. The term "JP-A" used herein refers to a published unexamined patent application, and the term "JP-B" refers to a published examined patent application.
Many nonlinear optical materials are described in JP-A-62-59934, JP-A-63-23136, JP-A-63-26638, JP-B-63-31768, JP-A-63-163827, JP-A-63-146025, JP-A-63-85526, JP-A-63-239427, JP-A-1-100521, JP-A-64-56425, JP-A-1-102529, JP-A-1-102530, JP-A-1-237625 and JP-A-1-207724.
As already mentioned before, high nonlinear susceptibility in a molecular state is not the sole condition for compounds to be useful as quadratic nonlinear optical materials and it is also essential that the arrangement of molecules in an aggregated state have no inverse symmetry. However, it is extremely difficult in the state of the art to predict the molecular arrangement of compounds and, in addition, the probability of occurrence of suitable compounds among all organic compounds is not high.